Abstract
Protein kinase A (PKA) is one of the most studied eukaryotic signal transducers and a key target of the second messenger cAMP. Kinetoplastids, a branch of early diverging eukaryotes, possess homologues of catalytic and regulatory PKA subunits that, unusually, are insensitive to cAMP with uncertainty about the endogenous activating ligand. While previous evidence has implicated both cAMP and PKA in the regulation of flagellar motility in kinetoplastids, the specific contributions of the two regulatory (PKAR) and three catalytic (PKAC) subunits of PKA remain unclear. Here we conducted a systematic study combining reverse genetics and microscopy analysis to investigate PKAC and PKAR subcellular localisations, pairing preferences, and the effect of PKA gene deletions on swimming speed and flagellar waveforms in Leishmania mexicana promastigotes. LmxPKAC1, LmxPKAC2 and LmxPKAR1 are enriched in the flagellum and upon detergent extraction, the LmxPKAR1 and LmxPKAC1 signals remained associated with the cytoskeleton. The flagellar LmxPKAC1 and LmxPKAC2 signals were, however, greatly diminished by removal of LmxPKAR1, consistent with an anchoring function for the R-subunit. Previous work identified the Streptomyces antimetabolite toyocamycin, activated the divergent PKA of Trypanosoma brucei. The patterns of sequence divergence within the phosphate binding cassettes of LmxPKAR1 and LmxPKAR3 suggest both are divergent from T. brucei. We therefore tested the effect of toyocamycin and a similarly structured compound of the purine metabolism pathway, inosine, on the localisation of LmxPKAC1 and found that the LmxPKAC1 mNG signal was lost from the cytoskeletal fraction following treatment with either toyocamycin or inosine. LmxPKAC3 and LmxPKAR3 were localised to the cell cortex. Upon removal of LmxPKAR3, LmxPKAC3 was lost from the cortex and became distributed throughout the cell, including an increased presence in the flagellum. Assessing motility phenotypes, we found this increase of LmxPKAC3 signal in the flagellum correlated with an increase in swimming speed. Whereas the deletion of LmxPKAC3 resulted in flagella beating at significantly reduced frequencies and a resulting decrease in population swimming speed. Moreover, LmxPKAC1 null mutants showed a significantly reduced swimming speed and cells were unable to produce symmetric flagellar waves. Taken together these data indicate that the divergent PKA pathway in Leishmania modulates flagellar motility.
Author Summary Many cells use whip-like cellular appendages, called flagella, for swimming. Regulating the rhythm and strength of the flagellar beat is important to determine the speed at which cells swim and in what direction they move. How this regulation is accomplished is not fully understood. Here we examined the role of protein kinase A (PKA) in flagellar motility of the unicellular parasite Leishmania. Leishmania have three catalytic PKA proteins, which transduce upstream signals through protein phosphorylation, and two regulatory PKA proteins that serve to anchor the catalytic subunits to specific locations within the cell. We show that two catalytic and one regulatory subunit are part of the flagellar cytoskeleton, while the other regulatory subunit docks its catalytic subunit to the cytoskeleton surrounding the cell body. Removal of individual PKA proteins perturbed the flagellar beat in different ways. We found that cells that lacked the catalytic subunit PKAC1 could only beat their flagella in an uncoordinated manner and as a consequence swam more slowly. These findings suggest that Leishmania PKA proteins are part of a pathway that regulates flagellar beating.
Competing Interest Statement
The authors have declared no competing interest.
